Conventionally, a plasma processing is carried out on a wafer serving as a substrate in a vacuum chamber, an inner wall thereof being coated with sprayed ceramic such as yttrium oxide (Y2O3) (yttria) and aluminum oxide (Al2O3). In General, since ceramic has a high reactivity with water molecules or moisture in the air, when the chamber is opened to the atmosphere by opening its lid during a regular examination or wet cleaning thereof, the water molecules may get attached to, for example, the inner wall of the chamber or an upper electrode therein.
FIG. 5 is a graph showing measurement results of an atmosphere in a plasma etching chamber (etcher) for performing a plasma etching process on the wafer, obtained by a quadropole mass spectrometer (QMS), wherein a vertical axis represents the QMS count and a horizontal axis represents the mass number.
The measurement is made right after closing the lid of the plasma etching chamber which has been opened to the atmosphere. Further, the plasma etching chamber is made of aluminum, and its inner wall is coated with alumite.
FIG. 5 shows that a peak due to molecules having a mass number of 18, which is the mass number of water molecules, is the highest, and it can be deduced therefrom that there are a large amount of water molecules present in the plasma etching chamber right after the lid has been closed. The large amount of water molecules may cause the following problems:
1) To create a vacuum inside the chamber, the inside thereof must be exhausted, and the presence of the water molecules therein increases the time required to reach the required vacuum and reduces an efficiency of a processing apparatus;
2) During a metal film forming on a wafer in a chamber of a CVD apparatus, the presence of water molecules inside the chamber may cause a number of abnormalities such as forming of an oxide film, peeling of film layers from the wafer surface and increasing of wafer surface resistance;
3) In etching of the oxide film, an etching rate of a wafer lot right after the chamber's lid is closed is different from that of the wafer lot in the chamber whose inside has become stable after a specified time period;
4) When the wafer is etched by using plasma generated by a plasma generation gas containing fluorine, water molecules in the chamber react with the plasma generation gas to form fluoric acid, the fluoric acid, in turn, corroding the inner wall surface, generating peeled particles.
5) An abnormal discharge occurring due to the presence of water molecules inside the chamber may damage the wafer and facilitate a generation of the peeled particles.
To solve the above-mentioned problems, there is known a technology wherein HCl, BCl3, DCP (dichloropropane) and DMP (dimethylpropane) are introduced into the chamber (etcher) whose inner wall is coated with alumite to accelerate the removal of water molecules in the chamber (see, e.g., Journal of Vacuum Science and Technology, A14, 1266 (1996)).
In the technology, however, HCl, BCl3, DCP and DMP do not readily react with water molecules. Thus, all of the water molecules emitted in form of an out gas from pores in the alumite cannot be processed, and as a result, although HCl, BCl3, DCP and DMP are introduced in the chamber, it is difficult to accelerate the removal of water molecules in the chamber (etcher).